turning

A woman machinist operating a gun turning lathe during World War II.

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Turning is the process whereby a single point cutting tool is parallel to the surface. It can be done manually, in a traditional form of lathe, which frequently requires continuous supervision by the operator, or by using a computer controlled and automated lathe which does not. This type of machine tool is referred to as having computer numerical control, better known as CNC. and is commonly used with many other types of machine tool besides the lathe.

When turning, a piece of material (wood, metal, plastic even stone) is rotated and a cutting tool is traversed along 2 axes of motion to produce precise diameters and depths. Turning can be either on the outside of the cylinder or on the inside (also known as boring) to produce tubular components to various geometries. Although now quite rare, early lathes could even be used to produce complex geometric figures, even the platonic solids; although until the advent of CNC it had become unusual to use one for this purpose for the last three quarters of the twentieth century. It is said that the lathe is the only machine tool that can reproduce itself.

The turning processes are typically carried out on a lathe, considered to be the oldest machine tools, and can be of four different types such as straight turning, taper turning, profiling or external grooving. Those types of turning processes can produce various shapes of materials such as straight, conical, curved, or grooved workpiece. In general, turning uses simple single-point cutting tools. Each group of workpiece materials has an optimum set of tools angles which have been developed through the years.

The bits of waste metal from turning operations are known as chips (North America), or swarf (Britain). In some areas they may be known as turnings.

Turning operations

Turning is one of the most basic machining processes. That is, the part is rotated while a single point cutting tool is moved parallel to the axis of rotation (Todd 1994, p. 153). Turning can be done on the external surface of the part as well as internally (boring). The starting material is generally a workpiece generated by other processes such as casting, forging, extrusion, or drawing.

Facing is part of the turning process. It involves moving the cutting tool at right angles to the axis of rotation of the rotating workpiece (Todd 1994, p. 153). This can be performed by the operation of the cross-slide, if one is fitted, as distinct from the longitudinal feed (turning). It is frequently the first operation performed in the production of the workpiece, and often the last- hence the phrase "ending up".

Parting is used to create deep grooves which will remove a completed or part-complete component from its parent stock.

Grooving is like parting, except that grooves are cut to a specific depth by a form tool instead of severing a completed/part-complete component from the stock. Grooving can be performed on internal and external surfaces, as well as on the face of the part (face grooving or trepanning).

Drilling is used to remove material from the inside of a workpiece. This process utilizes standard drill bits held stationary in the tail stock or tool turret of the lathe.

Screw cutting both standard and non-standard screw threads can be turned on a lathe using an appropriate cutting tool. (Usually having a 60, or 55° nose angle) Either externally, or within a bore.^[1] Generally referred to as single-point threading.

Lathes

A lathe is a machine tool used principally for shaping pieces of metal, wood, or other materials by causing the workpiece to be held and rotated by the lathe while a tool bit is advanced into the work causing the cutting action. Lathes can be divided into three types for easy identification: engine lathe, turret lathe, and special purpose lathes. Some smaller ones are bench mounted and semi-portable. The larger lathes are floor mounted and may require special transportation if they must be moved. Field and maintenance shops generally use a lathe that can be adapted to many operations and that is not too large to be moved from one work site to another. The engine lathe is ideally suited for this purpose. A trained operator can accomplish more machining jobs with the engine lathe than with any other machine tool. Turret lathes and special purpose lathes are usually used in production or job shops for mass production or specialized parts, while basic engine lathes are usually used for any type of lathe work.

Workholding Methods

• Three-jaw chuck: Used with a variety of diameters of round stock. It is a self centering chuck. it is used to hold only round jobs.
• Four-jaw chuck: Used mainly for holding irregular shapes.
• Collet chuck: Primarily used for small round workpieces.
• Faceplate: A faceplate, drive dog, and mandrel may be used to turn workpieces such as gearblanks.
• Drive center: Use hydraulic or spring-loaded teeth that "bite" into the end of workpieces and can be used when the entire length of the workpiece must be machined.

Tooling

The various angles, shapes, and sizes of a single-point cutting tool have direct relation to the resulting surface of a workpiece in machining operations. Different types of angle such as rake angle, side rake angle, cutting-edge angle, relief angle, nose radius exist and may be different with respect to the workpiece. Also, there are many shapes of single-point cutting tools, such as V-shaped and Square. Usually, a special toolholder is used to hold the cutting tool firmly during operation.

Material removal rate

The material removal rate (MRR) in turning operations is the volume of material/metal that is removed per unit time in mm³/min. For each revolution of the workpiece, a ring-shaped layer of material is removed.

MRR = pi×D_avg×d×f×N where

D_avg: Average diameter
N: Rotational speed of the workpiece
f: Feed
d: Depth of cut

Dynamics of turning

The relative forces in a turning operation are important in the design of machine tools. The machine tool and its components must be able to withstand these forces without causing significant deflections, vibrations, or chatter during the operation. There are three principal forces during a turning process: cutting force, thrust force and radial force.

• The cutting force acts downward on the tool tip allowing deflection of the workpiece upward. It supplies the energy required for the cutting operation.
• The thrust force acts in the longitudinal direction. It is also called the feed force because it is in the feed direction of the tool. This force tends to push the tool away from the chuck.
• The radial force acts in the radial direction and tends to push the tool away from the workpiece.

Although it requires less-skilled labor, the engine lathes do need skilled labor and the production is somewhat slow. Moreover, it can be accelerated by using a turret lathe (In a turret lathe, a longitudinally feedable, hexagon turret replaces the tailstock. The turret, on which six tools can be mounted, can be rotated about a vertical axis to bring each tool into operating position, and the entire unit can be moved longitudinally, either manually or by power, to provide feed for the tools) and automated machines.

Time calculations

Diameter of workpiece (in.): D; Length of turning cut (in.): L; Length of facing cut (in.): W; Approach (in.): A; Feed Rate (ipr): F; Cutting Speed (sfpm): V; Retract Rate (ipm): r; Traverse Distance (in.): S; Traverse Rate (ipm): R;

(Todd 1994, p. 157).

See also

• Hard turning
• Surface feet per minute
• Woodturning

References

• "Lathe Introduction". MIT Lathe Introduction. http://electron.mit.edu/~gsteele/mirrors/www.nmis.org/EducationTraining/machineshop/lathe/intro.html. Retrieved 2008-10-27. 
• "Lathe Turning". http://www.idc-shanghai.com/Popular%20Technology/machining/Turning.htm. 
• Todd, Robert H.; Allen, Dell K. (1994). Manufacturing Processes Reference Guide.. New York: Industrial Press. 

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